You'll begin a new game. You can continue working toward your progress for trophies. All of your previous progress will carry over.

Additionally, in the pause menu, under options, there will be a new listing for items. Selecting items will allow you to change costumes.
The more barrels you managed to collect, the more costumes you will have. Barrel progress accumulates over multiple runs through the story,
allowing you to unlock all of the costumes by playing the story repeatedly.

You have to complete a playthrough to 'lock in' your barrels. If I play the first time through the game and collect 40 barrels, and then I
start a new game and collect 8 barrels, and then start a new game again without finishing the story, I'll lose those 8 barrels and go back
to 40. You can't just collect those first 9-10 barrels again and again to unlock everything.
However, you won't ever lose anything you've
unlocked, so if you get to 48 barrels to unlock the Horned Apparel, and then start over - while you will go back to 40 barrels, you will still
have access to the Horned Apparel costume, despite it requiring 48 barrels to unlock. Since you have 48 barrels at one point, the costume is yours.

To access any of these items, pause the game, select Options, and after completing the game the first time, there will be a new menu selection
for Items. Simply place a check mark by the costume you wish to use, or no check mark to go back to the default costume.

Items

On subsequent playthroughs you have access to a few new costumes and accessories for the boy and Trico.

The Badge of Honor
Earned by completing the game at least one time. The badge of honor will be worn around Trico's neck, like a dog license.
The badge of honor will continue to get bigger and fancier each time you complete the game.

Enduring Attire
Earned by collecting 16 barrels* and completing the game once. A white undershirt with a dark blue wrap with gold trim.

Homecoming Attire
Earned by collecting 32 barrels* and completing the game once. Inverse colors from the default costume, this outfit has a white
undershirt and an orange wrap with gold trim.

Horned ApparelEarned by collecting 48 barrels and completing the game once. Based off of Ico's costume, the colors are similar to the default
costume, with a purple pattern from Ico's poncho at the bottom of the outer wrap. This costume also adds a white bandanna wrapped around the boys
head, and a pair of close toed sandals on his feet.

Warrior's Clothes
Earned by collecting 64 barrels. Based off of Wander's costume in
Shadow of the Colossus, this has a blue undershirt with a
white wrap with blue trim. A blue cloth with the same pattern as Wander's poncho is worn around the waste, with pantlegs sporting a similar pattern
poking out below. Also includes Wanders open toed cloth wrapped sandals, and a blue headband worn under the hair, poking out at the forehead.

Dressed for the Fest Trophy

This trophy is earned simply by collecting 64 barrels and then trying on every different outfit for the boy.

Fine FeatherpainterEarned by collecting 96 barrels. Gives you a cute little glowing trico toy that you wear on your back.
This item is used in the same way as the mirror shield - and must be disabled in the game when you want to use the mirror shield instead.
Press or on the D-Pad to take the featherpainter out, and hold to use
it to shine a light. If you shine it at Trico, it will change the
color of his feathers wherever it hits him. You can cycle between colors by pressing and on the D-Pad.

Unfortunately, you don't have much control over the featherpainter. You won't be able to make any really intricate patterns. But if
you've been wishing that you could have a purple Trico, or maybe paint him to look like your own pet, or just give him a cool racing
stripe to make him faster** this is just what you're looking for.
* I'm guessing on the number of required barrels for these early costumes. I got 40 barrels on my first playthrough, and these costumes were unlocked.** Giving Trico a racing stripe will not actually make him faster. That was a joke.

Beast Friends Forever Trophy

Earning the Fine Featherpainter will take 96 barrels. There are 48 in the game, so it will take a minimum of 2 playthroughs to get it -
probably more, because you aren't likely to get every barrel on your first playthrough unless you're following the walkthrough on your first attempt
of the game. Once you have 96 barrels, you can equip the fine featherpainter by pausing the game, selecting Options, then Items, and putting a
check mark under Fine Featherpainter. Then simply use the Featherpainter the same way as you'd use the shield. Press and on the D-Pad to
cycle between colors. Shine the featherpainter at Trico to change his color. When you get most of Trico painted, you'll earn this trophy.